A prospective study of health effects of passive smoke exposure has been completed. In this study of over 48,000 adults, the mortality and cancer experience of nonsmokers who lived with smokers in 1963 was compared with that of nonsmokers who did not live with nonsmokers and with smokers. Study subjects ere residents of western Maryland who had participated in a special county-wide census in 1963. Data collected at that time on smoking histories or every member of a household were used to derive a measure of exposure to smoke at home for each participant in the study. The cohort was followed through 1975 using medical and vital records. Underlying and contributing causes of death were obtained for census participants who died in the county between 1963 and 1975. Cancer incidence was ascertained through a county- wide cancer register. Results suggest that nonsmokers who live with smokers are at increased risk for death from all causes, at increased risk for death from arteriosclerotic heart disease, and at increased risk for developing certain cancers when compared with nonsmokers who are not exposed to smoke at home. Ongoing analyses using this study include evaluation of the role of cigarette smoke exposure in the etiology of potentially hormonally mediated cancers such as the breast, cervix, prostate and colon. Data have also been used to describe characteristics of never-smokers who have passive smoke exposure at home.